Cationic froth flotation of iron ore



Patented Oct. 4, 1949 CATIONIC FROTH FLOTATION OF IRON ORE Fred D. De Vaney, Hibbing, Minn., asslgnor to ErieMini ng Compan poration of Minnesot y, Hibbing, Minn, a cora,

No Drawing. Application March 25, 1946, Serial No. 657,076

8 Claims. (01. 209-166) materials containing fine oxidic iron particles whereby markedly to depress or inhibit the floatability of the latter and hence to give a cleaner separation between the same and the flne silicious particles associated therewith.

It heretofore has been proposed to beneflciate silicious low-grade iron-bearing material by a process which, in essence, consists in subdividing the material to size of liberation, forming an aqueous pulp of the subdivided material, adding a relatively small amount of a readily water-dispersible cationic collector-without, or preferably with, added frothing agent-to a body of the aqueous pulp, and subjecting the resulting mixture to agitation and aeration in a froth flotation cell, whereby to develop a froth rich in silica and poor in iron, and separating the froth from the unfloated residue. Such a process is described and claimed in my copending now abandoned application entitled Beneficiation of iron ores, Serial No. 500,002, filed August 25, 1943. This "cationic" type of froth flotation process operates very well on pulps whose liberated oxidic iron particles are relatively coarse and which have eitherbeen freed from colloidal matter (slimes) or have had their colloidal matter rendered relatively innocuous by recourse to a flocculation treatment or suitable "cnditi0ning treatment. The application of this process, however, to pulps having a substantial content of fine oxidic iron particles has the disadvantage that undesirably large amounts of the cationic collector (usually a rather expensive chemical) are required: moreover, the resulting concentrates tend to run too high in silica and too much iron mineral is discarded in the tailings.

It is an object of the present invention to provide an improved cationic type of froth flotation process which is efiective for application to crude pulps of silicious iron-bearing material. Another inventive object is to modify the conventional cationic flotation process in such manner as to depress the iron mineral flnes, i. e., to inhibit the flotation of these fines.

It has now been found that the above disadvantages may be obviated, and that the above and other inventive objects may be realized by including in the charge to the flotation cell, e. g., by incorporating into the pulp a relatively small amount of a modified starch product (hereinafter to be more specifically described) and conducting the cationic froth flotation operation upon the resulting mixture. By so modifying the convention a1 procedure the iron contents of the resultant tailings are desirably lowered and the resulting concentrates are of higher grade and also contain a greater proportion of the total iron in the feed; moreover, this modified procedure permits a desirable economy in the amount of collector used per unit of ore treated, and also makes possible the selection of operable cationic collector from ,a larger group of the latter than was available under the prior practice.

By the expression modified starch product is here meant a starch, consisting mostly (i. e., about or better) of relatively large molecuied, branched components, which has been only moderately degraded, by one or another of known processes of degradation, to the point where (l) the starch granule structure has been substantially weakened or destroyed to allow good dispersibility in water, so that the starch product has acquired a. 5 to solubility in cold water, and

(2) the ferricyanide reducing value of the starch product lies between about 20 and about 250, optimum 35 to 80.

Relative to the first criterion, it is to be noted that modified starch products having less than a 5% solubility in cold water include most-\thinboiling starches, oxidized starches and socalled soluble starches. This limitation excludes about 80% of the very many odified starch products tested by me. The upper limit of 80% solubility is imposed because products with higher than 80% cold water solubility usually have suffered too extensive chemical degradation and are composed of, or contain a large proportion of, carbohydrate molecules too small in molecular size to function as depressors of the iron mineral fines in the cationic flotation process.

In connection with the second criterion above, it is noted that a good measure of the extent of chemical degradationor the degree to which the starch molecule has been broken or hydrolyzed into smaller fragments-is the test described by Farley and Hixon in "Indus. & Eng. Chem., Anal. Ed., vol. 13, p. 616 (1941). With this test, which primarily is a means for determining aldehyde groups, it is possible to correlate average molecular size with the ferricyanide reducing value of the product. The ferricyanide reducing value of unmodified starch is 10, and that of glucose is 2800; ferricyanicle reducing value is proportional to extent of hydrolysis up to a value of about 1500. Many of the modified starch products found to be very eifective as depressors of iron mineral fines have ferricyanide reducing values of 80 or less, whereas some modified starch products, commercially styled dextrins," found to have only slight depressing effect have ferricyanide reducing values of the order of 250 and higher.

dextrose) also is a desirable quality of a modified starch product effective as a depressor in the present relation. A substantial sugar content is indicative of too extensive degradation of the starch molecules and is reflected in high ferricyanide reducing value of the modified starch product.

Ancillary to the above criteria, I have found that the depressor effect of an operable modified starch product is to be attributed to its content or branched components, and that the linear components of the starch are without the depressor efiect and probably are to be assessed as were diluents therein. These facts have been established by me on the basis of experimental data involving the use of separated fractions of branched components and linear components and use of admixtures of the two. Modified starch products prepared from corn starch (e. g., the product known as Amioca) from "waxy maize" corn, composed almost entirely of branched molecules, have higher depressor efliciency than modified starch products similarly processed and having the same ierricyanide reducing value and same cold water solubility but derived from corn starch containing 72-75% of branched molecules.

Included within the scope of the above defininot exhaustive of the latter, are: some British gums or heat dextrins, and an acidified starch or acid-modified starch prepared by heating the unmodified starch with a small amount of mineral acid in dilute aqueous solution, e. g., in the proportions of about 5 grams of the unmodified starch and 100 ml. of water containing about 0.5 ml. of concentrated sulphuric acid, the heating being continued for several minutes, without the necessity of boiling, until the dispersion became relatively clear.

In connection with this latter product, it is noted that the resulting solution is used in the flotation procedure directly, that is to say without neutralization. The effect of the acid modification is to weaken the granule structure (as shown by its very low paste viscosity) and to disperse it in the water with relatively little chemical degradation of the molecules: it still gives a blue iodine color, and its ferricyanide reducing value falls within the range -50. e

Other specific modified starch products answering the above criteria and found to have excellent depressor eflfect in the present relation are:

(a) A modified corn starch product, of the "heat dextrin type, having a cold-water solubility of 70% and a ferrlcyanide reducing value of '76;

(b) A modified starch, of the "soluble starch" type, having a cold-water solubility of 10% and a ferricyanide reducing value of 76; and

(c) A modified corn starch product, of the British gum type, having a cold-water solubility of about and a ferricyanide reducing value of about '75.

The effect of the above described acidified starch is shown in the batch tests data of Table No. 1 following, in which the starting material employed, the identities and relative amounts of collector and of frother, and the conditions of operation were as follows: The acidified starch was a product produced by heating corn starch in water containing about 1% H01, based on the weight of the starch, for about 15 minutes. The pulp was conditioned with the.acidified starch for about 10 minutes, prior to carrying out the batch flotation step. In this series of batch operations the starting material operated upon was a washing plant tailing from the Danube mine washing plant. This tailing product analyzed 35.2% Fe (mostly hematite, with a small amount of limonite) and a gangue composed largely of chert and quartz. This starting material was finely divided (minus 60 mesh) as received, and was not further subdivided; it contained a substantial amount of ultra-fine particles (slimes). The starting material was not deslimed or subjected to a flocculation treatment. The separations were efiected in a standard mechanically agitated flotation cell, 500 grams of the starting material being used per batch with sufiicient water to yield a pulp containing about 25% solids. In each test there were used:

(a) As cationic collector, 0.4 lb. a cationically acting amine collector, more particularly described below, per ton ore, and

(b) As frother, 0.104 lb. methyl amyl alcohol per ton ore.

ments during the carrying out of the flotation in step.

TABLE 1 Effect of acidified starch in amine flotation-- Danube tailinys Acidified ssay, Percent Starch, lb./ton

Percent Percent of Tom Product weight Iron Silica Tailings Middlings Concentrates...

Total...

Tailings .4 Middlings Concentrates..-

Total..."

{Tailings .8. Middlings Concentratea.

Total.. 36. 12

Concentrates...

Total..."

{Tailings 2.0 Middlings Concentrates" Total...

{Tailings 8 ass 8 see Concentrates...

Total...

It will be observed that improvement (in grade and in quantity of concentrate) followed the progressive increases in amounts of acidified starch employed, up to an optimum of about 2.0 lb./ton (between 1.6 and 3.0 lit/ton) of the acidifledstarch-fl' Astheamount of acidified starch exceedsitheioptimum' the silica particles j gare also inhibited from floating and tend to con-'. taminatetheconcentrate;

* sota and locally known as taconite and oxi- 'di zed taconite) it is desirable if not absolutely necessary that. the circuit be maintained alkaline (e. g., at an; alkalinity equivalent to a pH of '8 or morei On the other hand,'it has been found that best. results in operating on'certain washing plant tailings follow adjustment of the alkalinity of the circuit to neutral or only very slightly alkaline '(pH7-"7-.5). This latter observation is illustrated ln;the. data, of Table 2 following, of

batchtests performed on a low grade washingv plant tailing conditioned with about 2 lbs./ton of acidified starch;

TABLE 2 that the only difierence in conditions is that of the alkalinity of the pulp (i. e., pH 7.5 in the fol-'- lowing test against pH 8 in Table 1):

contiriildlisite'sts (instead of the vefb h Danube washing plant tailings tests), madeiatithe rateof 1- ton per hour, in" 4 I commercial" sized-float'machine, have dein'on- A a P t strated that metallurgical results similar to those SSW, amen recited abovearesecurable with a collectorcon- 10 r m 662% gii t zfl l rog sumption, and-with a consumption of "acidified r f. mm $11108 starchj of only-about one-half those amounts I employedinihe batchteste Thusfin continuous tiiifit;::::""" 32?? 333 533% tests paralleling the batch tests recited in Table 1, Y Concentrates. 41. 4c cs, 01 5.04 as. 91 optimum results were secured with a'collector moo 3M0 moo consumption of' 0.18 lb./ton and 1.0 lb./ton .of "acidified starch: as little as 0.2 lb./ton of the "acidified starch exerted a real improvement in a the grade and quantity of the concentrate. I R nts:

It has been iound that best results are attained o by adjusting the alkalinity of the circuit, con- Collector recitedabove-OA lb. per ton; taining the-modified starch product, to suit the Acidified 'starch-2.0 lb. per ton; pa ticu rr in t at h t has been Methyl amyl a1cohol.'104lb. per ton. found that in operating on certain Ierruginous erthose found in northeastern Minner The "modified starch product may be a commercial, soluble jstarch having a cold water solubility greater than 5% and not greater than 80% and having .a fe'rricyanide reducing value iallingWithinthe range 20-250. Illustrative are the following data of a batch test using a soluble ducing value of 76. In this test the starting material was identical with that described in reference to Table 2 above, and the conditions were identical withthose observed in the second test of Table 2, with the exception that 1.0 lb./ton of Effect of alkalinity in amine flotation a danube tailings using "acidified starch Add 1mm I P t Assay, Percent, Percent or. ercen pH-Pulp Product of Total Added 1 V6 eight hon silica Iron Tailings 37. 92 6. 35 14. Sulphuric acid 5.9-6. 1 Middlings 40. 12 5. 70 13. 59 Ooncentl'ates. 21. 96 65. 16. 47 72. 11

Total 100. 00 16. 79 100. 00

Tailings 59. 78 4.23 15. 33 None 7. 0-7. 3 Middlings 22. 58 13. 02 17. 82 Concentrates. 17. 64 j 62. 52 5. 76 66. 85

Total 100.00 16.50 1 100.00

\ Tailings..- I 53. 37 14.42 Soda Ash; 7. 7-8.0- Middlings 28. 21 16.70

0oncentrates 18. 42 68. 88

Total. 100.00 16. 29 100. 00

- Tailingsl. 43. 4. 74 12. 72 Do 8. 7-9.0 Middlings 38. 20 10. 20 24. 09

Concentrates' 18.40 55. 17.22 63. 19 f Total- 100. 00 16. 19 100. 00

Conditions of test: the commercial soluble starch was used-in place f2. 1b. 0 oft "a 'd'fi h": ore charge 500 gm. 0 0 /t n he 01 1 ed starc I Collectorthe previously recited amine mix- Assay Percent ture, 0.4 lb./ton ore Product Percent Percent of Frother-methyl amyl alcohol, 0.104 lb./ton Weght Iron a Tta1 ore Acid oralkalraddition as-indicated. gi gg l Si 2g 22g; The improved. results obtained by using a 03110852555553:31:33: 12:70 63:00 3933 nearly neutralpulp may be seen by comparing the IT 1 10 00 16 67 following test with the fifth test of Table 1 noting 8 I mm With some ores, the modified starch product can be added directly to the head of the flotation circuit, while with other ores it is advantageous to condition'the pulp with the modified starch product forsome minutes prior to the actual flotation operation. 4

While in the above illustrative specific examples the cationic collectors employed were primary aliphatic amines such as mixtures of watersoluble'forms of primary aliphatic amines corresponding in alkyl chain length to fatty acids of coconut 011, above referred to)\and commercial dodecylamine hydrochloride, it is to be understood that the above described modified starch products exert their selectivity-enhancing (specificall y, iron oxide-depressing) properties in the presence of cationic collectors generally.

The following specific example illustrates the applicability of the principles of the invention in the flotation of'silica from hematite in a batch test using a well-known quaternary ammonium compound as collector.

The feed was a deslimed washing plant tailing from Danube mine above referred to. In size it was substantially all finer than 65 mesh, and was not further subdivided. In test 682-A, there was used as collector a quaternary ammonium chloride derivative from myristic acid. The collector, used inthe form oi a 2% aqueous solution,'was added in three equal portions to the rougher cell and in one further portion to the cleaner cell. Q As the frother there was used a higher alcohol product, which frother was added to the cleanercell in a single addition. In test 682-18 the same collector and frother were used,

but there-also was used, as iron oxide depressor, the above referred to modified starch product having a cold-water solubility of 70% and a ferricyanide reducing value of 76.

Conditions of test:

Ore charge-500 gm.

Collector-same quaternary compound-0.32 lb./ton

Frother -same higher alcohol-0.05 lb./ton

Depressor-same modified starch product as was used in test No. 682-A-2.5 lb./ton

Pulp density-25% pH of pulp-7.1.

It will be noted that while in both of the above tests high-grade concentrates were made, the rougher overflow (or tailing) contained, in test 682-A, 7.43% of iron, whereas in test 682-3, in which themodified starch product was used, the iron content of the rougher overflow fell to only 3.39%: also, that when the modified starch product was used the weight recovery of concentrates was 21.87%, as contrasted with only 15.11% when the modified starch product was omitted. With use of the latter agent, the overall iron recovery was 79.82%, whereas it was only 56.93% when the modified starch product was omitted. As will be obvious, the modified starch product definitely sharpened and improved the separation. It will be noted also that although somewhat more collector was used in the B test than in the A from which fact it was to have been predicted that more iron would be carried into the froth in --B than in A-there actually was carried into the froth only one-half as much of the iron as in the --A test, this being explainable only on the basis of the strong depressing effect of the modified starch product on the flotation of the hematite particles. 1

The previously given examples have shown the application of this improved process to the concentration of washing plant tailings. It has been found, surprisingly, that the improved process The batch tests data of test No. 682-A follow: 40 admirably is adapted for use in the beneficiation Assay Per Cent Must Wt. in- Per Cent Per Cent oi Grams Weight Total Iron Iron Silica Units Underflow 75. 00 15. 11 63. 31 5. 66 9. 57 56. 93 Cleaner Ovarflow 29. 80 6. 00 23. 01 l. 38 8. 21 Rougher Overflow- 391. 70 78. 89 7. 43 5. 86 34. 86

Total... 406.50 100.00 16. 81 6.66 16.81 100.00

Conditions of test: of ferruginous cherts such as those deposits Ore charge-500 gm. Collector-the above-recited quarternary ammonium compound-0.28 lb./ton Frother-a higher alcohol-0.05 lb./ton Pulpdensity-25% pH of pulp-7.1.

The batch tests data of test No. 682-3 follow:

known in northeastern Minnesota as taconites and oxidized taconite." The concentration of these latter materials is much more diflicult than the concentration of, say, wash ores or washing plant tailings in which the particles of iron oxide and silica are relatively free, one from another, at say minus mesh. In the taconites the locking between the minerals is much more intimate and even with 200-mesh grinding there is a conammonium operation.

TABLE 3 Effect of modified starch product addition on amine flotation of an oxidized taconite sinks. If the separation is not sharp, there is a tendency for the finer iron minerals tobe carried over in the froth and for the coarse silica particles to remain in the underfiow together with the more granular portion of the iron oxides.

As will be noted in the following comparative sizing analyses, the use of the aforesaid modified starch product in a cationic flotation operation desirably improves the separation of iron mineral from the finer material (slimes) of the ore pulp. These sizing analyses were made of the tailings (froth product) of two tests identical except for the fact that inone case, "test B, modified starch product was used, whereas in the other, "test A, the modified starch was omitted. Both A and B were continuous mill tests made with a 6-ce1l #12 Denver Sub-A flotation machine, the

chloride, collector; 0.05 lb./ton methyl amyl alcohol, frother;

Modified starch product addition as indicated in above table. Alkalinity in all tests of this table, pH 8.2.

From the above table it is evident that use of the aforesaid modified starch product made it possible to secure cleaner concentrates and simultaneously to increase the recovery; As the amount of starch product was increased the inhibiting A P m t feed in each case being at the rate of 1 long ton Asildififd P d t Percent Pe e rgoi per hour. In both tests there was used as cationic g; v sh Iron silica Iron 2 collector 0.182 lb./ton of the mixed amines product used in the tests reported in Table 1 7 above, and as frother 0.074 lb./to of a comma:- 4. 24.35 1 None {Effifi- 23,31 43,09 cial h1gher alcohol frother. In est B the ore Concentrates-" 16.09 53-99 17-81 32-56 pulp was conditioned, for 11 minutes prior to 100,00 2M9 mm 25 flotation, with 1.14 lbs/ton of the aforesaid I I 9 Tailings 50.02 11. a 22.30 acidified starh' V V {tiil'iiizaz 338% 233% 1%, Sim? analysis 11mm" tunings Total"--- 100.00 26.15 100.00, TmA TestB .0 {ta iii? mgs Percet Pe t P tP A Concentrates--- v 15.68 59.86 8.62 35.41 f?" j if elfient Total 100. 00 26.50 100. 00

- T if 4 3 9 79 1 ,64 .93 0. 61 2.77 2.0----; .--{Nfid d l% gs 31.60 21.00 32.70 iPg H2 Concentrates- 23.04 58. 55 11.11 50.57 +m0 62 44 12:49 98 s1 '1 ,00 2 100,00 11-72 5 54 Total 100.00 0.05 100.00 4.38 Reagents: A

From the above data it will be noted that al- 500 g. Ma orca taconite at minus 100 mesh not previously deslimd; though the acidified starch tends to prevent the 1b /tn commercial dodecylamine hydrm floating of the iron mineral at all sizes the effect is most marked on the material finer than 200 mesh. The chief advantage of using the modified starchproduct is that it exerts a marked depressingefiect on the fine iron minerals which have a tendency to come up in the froth because of their fine size. Accordingly, in most cases desliming is'obvi'ated by the use of the modified starch product, although in some instances it may be economically advantageous preliminarily to deslime the pulp.

effect of the reagent on the flotation of the hematite could be judged by noting the progressive increase in the weight of the concentrate and the progressive decrease in the grade of the tailings (froth product). g

The above data are illustrative of the applicability of the improved process of the presentinvention to difiicultly concentratable ferruginous cherts and slates and other silicious iron-bearing materials generally.

From a consideration of the role played by the aforesaid modified starch product, it is concluded that an important function of the material is that of aninhibiting agent, tending to prevent thefiotation of the iron oxide minerals.' This is particularly striking in the eifect of preventing the flotation of the more finely divided iron oxide particles (e. g., minus 200' mesh). In any froth flotation process the separation tends to be a "size" separation, in that there is a natural tendency for the finer material to be segregated in the froth while the coarser material jubility of from 5 to acids of the alkyl aryl type, gum tragacanth,-

gelatin, gum acacia, casein, glue, and water-soluble sugars (e. g., dextrose and maltose), do not answer the hereinbefore-recited combination of criteria (i; e., do not have a ferricyanide reducing value between 20 and 250 and a cold water soland do not give any improvement in the cationic froth flotation of gangue from oxidic iron mineral; in most cases their presence in the circuit is positively harmful rather than helpful. Thus, the presence of 1.0 lb./ton of sucrose in the circuit was found to impair rather than enhance the selectivity of a commercial collector consisting essentially of a mixture of primary aliphatic amines corresponding in alkyl chain length to fatty acids of coconut oil, in water soluble form, as did the presence of 0.5 lb./ton of gum tragacanth. Illustrative of the indifierent or even positively harmful effects resulting from the presence of raw corn starch in the pulp circuit are the batch tests data recited in the following table:

TABLE 4 Erica? of raw corn starch in amine flotation- Danube tailings Assay Percent Raw Percent Starch, Product of Total lbs/ton Iron I silica Iron Tailings 27. 43 12. 88 9. 98 None Middiings 38. 97 34. 25 37. 77 Conccntrates-. 33. 60 54. 96 l8. 17 52. 25

Total. 100.00 35. as 100.

Tailings 24. 46 9. 80 6. 65 2 Middlings 36. 78 .31. 27 31. 86 Concentrates... 38. 76 57. 24 15. 20 61. 49

Total. 100.00 3e. 09 10c. 00

Tailings 25. 04 9. l2 6. 33 4 Middiings 29. 21 27. 35 22.

Conceutrates.. 45. 76 56. 33 16. 44 7i. 50

Total- 100.00 38. 04 100.00

Tailings 22. 63 9. 93 6. 24 .8 Middlings 26. 27 25. 23 18. 37 Concentrates... 51. 10 53. 24 20. 53 75. 39

Total- 100.00 36. 08 100. 00

Tailings 22. 52 5. 93 1. 6 Middlings 28. 28 23. 65 18. 61 Concentrates..- 4Q. 20 55. 13 18. 35 75. 46

Total. 100. 00 35. 94 100. 00

Tailings 29. 55 7. 66 6. 22 2. 0 Middlings 30. 26 35.23 29. 32 Concentrates... 40. 19 58. 31 13. 08 64. 46

Total. 100. 00 36. 35 100. 00

Tailings 25. 02 s. 15 5. 74 3. 0 Middlmgs 32.06 29. 68 26. 74 Concentrates..- 42. 92 55. 94 17. 00 67. 52

Total. 100.00 35. 56 100. 00

Conditions of tests:

Ore charge-500 gm. pH maintained at 8.0

0.4 lb. a commercial collector consisting essentially of a mixture of primary aliphatic amines corresponding in alkyl chain length to fatty acids of coconut oil, in water soluble form, per ton ore -.104 lb. methyl amyl alcohol per ton ore The raw starch was raw corn starch which had been heated in water to form a hydrous jelly.

By a "ton of ore is here meant 2240 pounds of ore which has been dried to constant weight at 105 C.

It is to be understood that the above recitations of particular cationic collectors and of particular frothing agents are not to be considered as limitative of the scope of the present invention, but merely as illustrative of the latter.

While the improved cationic flotation process of the present invention has been described herein with particular reference to the beneficiation of washing plant tailings, it is of course to be understood that this process is not restricted to this latter starting material, and that the same is applicable in the beneficiation of high grade and low grade iron ores generally, stock pile lean ores, and tailings from gravity plants. I contemplate the incorporation of the present process in plants which employ gravity, washing, jigging, floatand-sink, magnetic separation or any other known 12 iron ore-concentrating method, as an additional step in the recovery 01 iron values from natural mill fines, reground low grade mill products and other iron-bearing materials of too low grade to be called iron ores."

This application contains subject-matter in common with my application Serial No. 505,697, filed October 9, 1943, and is a continuation-inpart of my application Serial No. 516,287, flied December 30, 1943.

I claim:

1. In the beneficiation of silicious iron-bearing material by a froth flotation procedure in which silicious gangue particles are floated from a pulp of the iron mineral, which pulp contains particles of various sizes including very fine oxidic iron particles, by the aid of a. readily water-dispersible cationic collector, the improvement which consists in effecting the flotative separation in a nonacid pulp circuit, in the presence of a modified starch product having a cold water solubility of from 5 to 80% and a ferricyanide reducing value of from 20 to 250, said modified starch product not containing more than 2% of water-soluble sugars and being substantially free from raw starch, whereby the fine oxidic iron particles are depressed into the underflow.

2. In the beneficiation oi silicious iron-bearing material by a froth flotation procedure in which silicious gangue particles are floated from a pulp of the iron mineral, which pulp contains particles of various sizes including very fine oxidic iron particles, by the aid of a readily water-dispersible cationic collector, the improvement which consists in eiiecting the flotative separation in a non-acid pulp circuit, in the presence of an aqueous solution 01 acid-treated starch having a cold water solubility of from 5 to 80% and a ferricyanide reducing value falling within the range 4 35-50, said modified starch product not containing more than 2% of water-soluble sugars and being substantially free from raw starch, whereby the flne oxidic iron particles are depressed into the underflow.

3. In the beneficiation of silicious oxidic ironbearing material by a froth flotation procedure in which silicious gangue particles are floated from an aqueous pulp of the iron mineral, which pulp contains silicious gangue and oxidic iron particles of various sizes including very fine oxidic iron particles, by the aid of an aliphatic amine collector in water-soluble form and a frother, the step of improving the selectivity of the flotative separation by suppressing the floatability of the very fine oxidic iron particles with respect to the floatability of the silicious gangue particles by the expedient of pre-conditioning a neutral to alkaline pulp of the material, in which pulp any alkalinity thereof is due to the presence therein of an alkaline-acting compound of an alkali metal, with an aqueous solution of a modified starch product having a cold water solubility of from 5 to 80% and a ferricyanide reducing value of from 20 to 250, said modified starch product not containing more than 2% of water-soluble sugars and being substantially free from raw starch.

4. Process of recovering by cationic froth flotation an oxidic iron concentrate from a pulp of silicious iron-bearing material which pulp is composed of particles of various sizes including very fine oxidic iron particles, which comprises conditioning an aqueous slurry of the material with an aqueous solution of a modified starch product having a cold water solubility of from 5 to 250, said modified starch product not containing more than 2% of water-soluble sugars and being substantially free from raw starch, establishing an aqueous, neutral to alkaline flotation pulp of said conditioned material in which pulp any alkalinity is due to the presence of an alkaline acting compound of an alkali metal, subjecting the pulp to agitation and aeration while adding thereto a frother and portion by portion an aliphatic amine collector in water-soluble form, whereby oxidic iron particles including the very fine oxidic iron particles tend to be depressed into the underflow while the silicious gangue particles float, removing a silicious gangue froth product poor in oxidic iron particles of all sizes, and recovering a concentrate of oxidic iron from the underflow.

5. Process of beneflciating taconite and similar ferruginous cherts by a froth flotation procedure in which silicious gangue material selectively is floated from an aqueous pulp of the taconite consisting of silicious gangue and iron mineral particles including very fine iron mineral particles, which comprises maintaining the pulp at an alkalinity of at least pH 8.0, and effecting the flotative separation using a readily waterdispersible cationic collector and a frother in the presence of an aqueous solution of a modified starch product having a cold water solubility of from 5 to 80% and a ferricyanide reducing value of from to 250, said modified starch product not containing more than 2% of water-soluble sugars and being substantially free from raw starch, whereby very fine mineral particles are depressed into the underflow.

6. Process of benefloiating taconite and similar ierruginous cherts by a froth flotation procedure in which silicious gangue material selectively is floated from an aqueous pulp of the taconite 4 consisting of silicious gangue and iron mineral 14 particles including very flne iron mineral particles, which comprises maintaining the pulp at an alkalinity of at least pH 8.0 by the use of an alkaline-acting compound of an alkali metal, conditioning the alkaline pulp with an aqueous solution of a modified starch product having a cold water solubility of from 5 to and a ferricyanide reducing value of from 20 to 250, said modified starch product not containing more than 2% of water-soluble sugars and being substantially free from raw starch, subjecting the soconditioned pulp to agitation and aeration while adding thereto portion by portion an aliphatic amine collector in water-soluble form and a frother, whereby very flne iron mineral particles are depressed into the underflow while the fioatability of the silicious gansue particles is not adversely altered, and recovering fromthe underflow an iron mineral concentrate.

'7. The improved beneflciation process deflned in claim 1, wherein there is employed an amount of the modified starch product corresponding to from about 0.2 to about 3.0 lbs/ton of the iron-bearing starting material.

8. The improved beneflciation process defined in claim 2, wherein there is employed an amount of the acid-treated starch corresponding to from about .4 to about 2.0 lb./ton of the iron-bearing starting material.

/ FRED D. DE VANEY.

REFERENCES CITED The following references are of record in the 

